How many KW/ph does your central heating use?

Thanks to the previous owner, my house has an oversized boiler (42kw) for 10 radiators. It’s also got reasonably good insulation all round. The bonus is the radiators reach full temp in 10mins from cold start and belt out a lot of heat. My combi boiler also modulates as soon as it reaches temp and I have a smart thermostat, so it’s not using 42kw an hour.. 

From many observations with my smart meter, I know that my boiler uses around 12kw to have the central heating and all radiators on for an hour, which doesn’t seem bad. However, I’ve often wondered how much this figure would be if I had a smaller and perhaps more suitably sized boiler, so please let me know what your figures and sizes are!
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  • markin
    markin Posts: 3,723
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    Its not oversized if you have 2+ bathrooms, its sized for hot water demand, The heating will be rated for 12-18 kwh if you look up the manual.

    Burning hot rads in 10 min could mean the flow temp is too high and so lower efficiency, if its a condensing boiler.
  • QrizB
    QrizB Posts: 13,664
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    I've got a 24kW regular boiler, which is also oversized for my house.
    Based on my smart meter data, I've known it run at 12kW when heating from cold, but once the system is warm it usually tickles along at 5-6kW, or a bit higher if the weather's frosty.
    I suspect I could replace it with a 12kW gas boiler or a 8-9kW heat pump and still get similar performance.
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  • RelievedSheff
    RelievedSheff Posts: 11,125
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    Ours is also oversized in a 5 year old new build. It's 32kW for a modest sized 3 bed detached.

    On the coldest of days we generally use about 30-35kWh per day. At the minute with it being milder we are using a lot less and averaging 10-15kWh per day.

    The house takes very little heating and it retains it's heat very well.
  • 10kwh electricity with ASHP on for 15 hours a day 

    However weather plays a part and we shot up to 22kwh used when temps were -1oC and tracker prices were good and left on on 24 hours.

    Sadly can't reveal the COP the system is 8 plus years old and didn't need it for grant payback.
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  • 32kw with a flow of 50C.  4 bed, 18 radiators, some 25 years old. Takes about 15mins to reach condensing mode and uses 6kw in the first 15mins.  Thereafter with the weather fairly mild uses average 4kw to maintain a set temperature of 20C.

    Yesterday used 50kw a previous cold day was 80kw.
  • Booker_j said:
    Thanks to the previous owner, my house has an oversized boiler (42kw) for 10 radiators. It’s also got reasonably good insulation all round. The bonus is the radiators reach full temp in 10mins from cold start and belt out a lot of heat. My combi boiler also modulates as soon as it reaches temp and I have a smart thermostat, so it’s not using 42kw an hour.. 

    From many observations with my smart meter, I know that my boiler uses around 12kw to have the central heating and all radiators on for an hour, which doesn’t seem bad. However, I’ve often wondered how much this figure would be if I had a smaller and perhaps more suitably sized boiler, so please let me know what your figures and sizes are!
    But havd you looked at the usage for the second or even third hours? And getting boiling hot that quick, sounds like you have the flow temp on full whack.

    My boiler is a WB 28kw. My flow temp is set to 4.25 out of 6 (did a trial last winter). Domestic hot water temp is fixed.

    This is my usage, in half hourly blocks. Heating on at 0730 - 0940.

    You can see how it's only running full pelt, right at the start. And your 42kw boiler sounds like it uses less than mine, =12.7 in that first hour (Figures from Bright app). 


  • Gerry1
    Gerry1 Posts: 9,769
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    edited 14 November 2023 at 4:12PM
    You've asked an ambiguous question hence you're getting two types of answer.
    There is no such unit as kW/ph.  It would mean kilowatts per per hour which is meaningless.  Don't worry, the confusion is widespread, even in the sticky article.
    The maximum power of a boiler is measured in plain kW, typically 16kW - 45kW.  Power is an instantaneous value although, as you've noted, some boilers can modulate down to a lower value.
    With current conditions you've found that to maintain your desired temperature the steady state power draw is 12kW.  (Full stop, not per hour or per anything.)
    You'd still need to draw 12kW steady state even if you had an 18kW boiler.  Within limits,  having an oversized boiler is no bad thing; it'll won't struggle in a big freeze and it could be useful if you extend the property.  However, if it's seriously oversized it may not always be able modulate low enough and then it will cycle on and off which will reduce efficiency.
    Some replies are referring to energy use in kWh which may not be what you were intending to ask.  A smaller boiler may use slighly less energy per day if it takes a long time to reach the set temperature.
    (Edited to emphasise that energy is measured in kWh.)
  • FreeBear
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    Gerry1 said: The maximum power of a boiler is measured in plain kW, typically 16kW - 45kW.  Power is an instantaneous value although, as you've noted, some boilers can modulate down to a lower value.
    My boiler, a Viessmann 050 30KW, will generate 3.2-25KW for heating at a flow temperature of 50°C or 2.9-22.4KW at 80°C. Total design load of ~10KW (8 radiators of various sizes) at a ΔT=50°C (so flow temp of 70°C), but am running at a flow temp of 50°C. Do not have any reliable data for gas consumption, but according to the Bright app, I used ~5.5KWh this morning to raise the temperature by 1°C.
    Modulation curve attached.

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  • tifo
    tifo Posts: 1,919
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    edited 14 November 2023 at 12:02PM
    My boiler (Worcester 30KW) seems to use a lot of gas, i don't know how to measure it (my IHD has never connected to the smart meter) and the boiler has a setting to show gas use but doesn't.

    I'd like to check my gas usage so how can i  do it?

    As an example, in summer i was topping up £60 a month for gas (some of it is for debt but i don't know how much, it's meant to be £15 a month but many times i get a letter stating the debt wasn't being taken and we'll start it now) and it has gone up to about £80 a month now. Last winter it went up to £200 a month for several months. 
  • tifo said:
    My boiler (Worcester 30KW) seems to use a lot of gas, i don't know how to measure it (my IHD has never connected to the smart meter) and the boiler has a setting to show gas use but doesn't.

    I'd like to check my gas usage so how can i  do it?

    As an example, in summer i was topping up £60 a month for gas (some of it is for debt but i don't know how much, it's meant to be £15 a month but many times i get a letter stating the debt wasn't being taken and we'll start it now) and it has gone up to about £80 a month now. Last winter it went up to £200 a month for several months. 
    Manually read your meter is how you do it
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